算法复习：栈和队列

用两个栈实现队列，用两个队列实现栈都要会

leetcode 面试题09. 用两个栈实现队列

#include<stack>
class CQueue {
public:
    stack<int> in;
    stack<int> out;
    int state=0;
    CQueue() { }
    void appendTail(int value) {
        if(state==1)
        {
            while(out.size())
            {
                in.push(out.top());
                out.pop();
            }
            state=0;
        }
        in.push(value);
        return;
    }
    
    int deleteHead() {
        if(state==0)
        {
            while(in.size())
            {
                out.push(in.top());
                in.pop();
            }
            state=1;
        }
        if(!out.size())
            return -1;
        int output=out.top();
        out.pop();
        return output;
    }
};

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue* obj = new CQueue();
 * obj->appendTail(value);
 * int param_2 = obj->deleteHead();
 */

 leetcode 232. 用栈实现队列

要求不能用stack，用list代替stack

注意：list相当于双向链表，基本操作：push_back() back() pop_back()  && push_front() front() pop_front()  && size() clear()

stack的基本操作：size( ) :返回栈中元素个数、top( ) :返回栈顶的元素、pop( ) :从栈顶取出并删除元素、push(e) :向栈顶添加元素e、empty( ) :栈为空时返回true

#include<list>
class MyQueue {
public:
    list<int> in;
    list<int> out;
    int state=0;
    /** Initialize your data structure here. */
    MyQueue() { }
    /** Push element x to the back of queue. */
    void push(int x) {
        if(state==1)
        {
            while(out.size())
            {
                in.push_back(out.back());
                out.pop_back();
            }
            state=0;
        }
        in.push_back(x);
        return;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(state==0)
        {
            while(in.size())
            {
                out.push_back(in.back());
                in.pop_back();
            }
            state=1;
        }
        if(!out.size())
            return -1;
        int output=out.back();
        out.pop_back();
        return output;
    }
    
    /** Get the front element. */
    int peek() {
        if(state==0)
        {
            while(in.size())
            {
                out.push_back(in.back());
                in.pop_back();
            }
            state=1;
        }
        if(!out.size())
            return -1;
        return out.back();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return (out.size()+in.size()==0);
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

leetcode 225. 用队列实现栈

队列queue的基本操作：front()显示队头、back()显示队尾、pop()删除队头、push()队尾插入、size()

#include<queue>
class MyStack {
public:
    queue<int> in;
    queue<int> out;
    /** Initialize your data structure here. */
    MyStack() { }
    int state=0;
    /** Push element x onto stack. */
    void push(int x) {
        if(state==1)
        {
            while(out.size())
            {
                in.push(out.front());
                out.pop();
            }
            state=0;
        }
        in.push(x);
        return;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if(state==1)
        {
            while(out.size())
            {
                in.push(out.front());
                out.pop();
            }
            state=0;
        }
        while(in.size()>1)
        {
            out.push(in.front());
            in.pop();
        }
        if(in.size()==0)
            return -1;
        state=1;
        int output=in.front();
        in.pop();
        return output;
    }
    
    /** Get the top element. */
    int top() {
        if(state==1)
        {
            while(out.size())
            {
                in.push(out.front());
                out.pop();
            }
            state=0;
        }
        while(in.size()>1)
        {
            out.push(in.front());
            in.pop();
        }
        if(in.size()==0)
            return -1;
        state=1;
        int output=in.front();
        out.push(in.front());
        in.pop();
        return output;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return (in.size()+out.size()==0);
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */